The present invention relates to a method and process for producing stretch wrap plastic film and in particular, the storage of such plastic film which has undergone stretching prior to storage on a roll.
In German laid open specification P3409117 there is disclosed a process for wrapping packaged goods in a web shaped stretch film wherein the film is provided in an already stretched state rolled up under tension; with the stretching taking place during the actual manufacture or as an intermediate step after processing of the film. In other words, in any film that has been stretched before storage, it is possible to store the film in its tensioned stretch state without allowing it to relax.
In this way, it is possible for the film to do work so that the retained tension in the film will act on a load or package with the film relaxing to a steady state when wrapped around a load or package. In the arrangement disclosed, it is necessary to avoid over stretching of the film and it may be necessary to apply adhesive to the stored film to avoid unwinding or recoiling of the film. Furthermore, it has been found that the relatively high stored tension in the roll may cause the core of the roll to collapse.
In Australian Patent No. 643902 there is disclosed a method and apparatus for making pre-stretched film from molten stock material, cooling the film, stretching the cooled film beyond the yield point and substantially relaxing the film to reduce the tension therein. It is disclosed that the major benefit achieved by this method and apparatus is that the film has been cold-oriented and relaxed to obtain a reduced web thickness and that the film does not need to be stretched as it is applied to an external configuration such as a load to obtain any savings of material. The film has merely to be tightened around a load upon application thereby providing better load containment than similar hand wrapped applications of prior art films.
In Australian Patent Application No. 55564/94 there is disclosed the provision of a roll of stretched plastics material film which has an embossed surface for trapping air so as to facilitate unwinding of the roll of stretched film for wrapping pallet loads. The film is totally relaxed prior to winding on to the storage roll.
The present invention seeks to minimise the perceived disadvantages of the known prior art and has as its primary objective the provision of a method for making a stretched film product where the stretched film is easier to use as a wrapping medium either for industrial, commercial or agricultural wrapping applications including for use in the making of silage.
Accordingly, the present invention provides a method of making stretch film including the steps of stretching the film beyond its yield point to an elongation greater than 100%, removing opposed edge zones of the stretched film to produce a film having a substantially uniform thickness, allowing the film to partially relax to release part of the elastic deformation component in the film to retain between about 20% and 80% of the elastic deformation, and storing the partially relaxed film for future use in the partially relaxed state.
Preferably the stretching of the film beyond its yield point may be in the range of 100% to about 400% of its original length. Conveniently this stretching may occur at or very shortly after the manufacture of the film in an extrusion machine so that the film is stored for later use in at least a partially stretched condition. It has been found that stretching of the film beyond its yield point causes distortions at marginal edge regions of the film which tend to form thickened areas compared with the main body of the stretched film web. These thickened edge portions cause a soft or spongy central zone once the film is wound up on a storage core that may inhibit maintenance of the retained degree of elastic deformation. Moreover, in the stored state the thickened edge portions tend to interact in overlapping film layers so as to inhibit correct dispensing of the film when it is desired to be used. It is therefore of importance to remove sufficient of the lateral ledge zones to produce a film of substantially uniform thickness. In this state the overlapping edge portions in the stored state do not interact with one another.
The present invention requires the retention of between 20% and 80% of the elastic deformation in the film after the stretching to at least 100% of its original length. More preferably between 40% and 70% of the elastic deformation is retained. The lower limit of 20% represents a minimum amount of useable elastic stretch being retained for effective wrapping of a load without the application of excessive further stretching loading during a wrapping action, that is during dispensing of the film from the stored state. The upper limit of 80% of the elastic deformation represents the maximum amount of stretch that can be retained while still being able to avoid damage to supporting cores on which the film is stored and telescoping of the film during storage.
In one specific preferred aspect, the film may be produced as a tube in an extruder of known construction and procedure and after cooling is stretched prior to cutting the tube to form webs of predetermined width and at predetermined retained memory or elastic deformation in the stretch procedure which can then be stored for later re-use.
The present invention also anticipates providing a stretch plastics material film made by any of the aforesaid methods.
In another preferred aspect, there is provided a method of wrapping a load with plastics material film including providing plastics material film stretched beyond its yield point which in a stored state retains between 20% and 80% of the film""s elastic deformation capability, and thereafter wrapping said film about said load. The load may be any product or material that requires wrapping but can include pallet wrapping, vegetation material during the production of silage, and bundle wrapping applications such as timber lengths, cans including drink cans and any other similar product. Wrapping of the load may be achieved by storing the plastics material on a roll capable of use in either a manually handled wrapping machine, or a powered wrapping machine of any conventional configuration. The advantage achieved is that the force required to establish a required elastic loading on the load to be wrapped is substantially reduced because of the retained elastic deformation levels in the plastics film.
In a still further aspect of this invention there is provided a method of dispensing a plastics material film including providing a plastics material film stretched beyond its yield point which in a stored state retains between 20% and 80% of the film""s elastic deformation capability, and thereafter dispensing said film from the stored state.
Conveniently in both the preceding aspects, the plastics material film prior to being stored in said stored state is stretched beyond its yield point in the range of about 100% to 400% elongation.